The system and method disclosed herein relate generally to controlling liquid level in a vessel and, more specifically, to controlling water level in a drum type boiler in a power plant.
Drum type boilers are the most commonly used boilers in power generation plants. Drum level trips are a frequent cause of trips and disruptions in a power station. In the power generation industry, drum level trips due to drum level control behavior during transient operations are the major cause of disruption in power generation and lead to huge loss of plant availability and revenue.
Control of drum water level is a challenging problem due to complicated dynamics of 2 phase flows, presence of waves, unknown heat and pressure disturbances, and load demands. Maintaining water level within limits is critical, as exceeding the limits will lead to trips or damage the equipment. The drum boiler has to operate in the presence of various unknown pressure and load (thermal) disturbances while maintaining water level within required limits. A drop in water level will cause thermal fatigue in the drum. An increase in water level significantly increases the possibility of water droplets entering a superheater/steam turbine and thus damaging the superheater/steam turbine.
Conventional approaches to control of water level typically include actuation of a feed water control valve that supplies water to the drum. However, such approaches to control water level are challenging for drum type boilers, especially during transient operating conditions due to inverse response exhibited by such systems. The magnitude of a controller response to inverse characteristics of the feed water flow demand is dependent on the drum boiler pressure. At lower drum boiler pressures, the effect of controller response to inverse characteristics of the feed water flow demand is exacerbated. Typical strategies to control drum boiler water level requires good measurement or estimation of steam flow rate, feed water flow rate, and water level. Incorrect measurements (such as due to faults or sensor malfunctions) may cause the plant to trip.
There is a need for a more effective technique for controlling a liquid level in a vessel, such as for example, controlling a liquid level in a drum type boiler, especially during transient operating conditions.